Science in the Classroom is a collection of annotated research papers and accompanying teaching materials designed to help students understand the structure and workings of professional scientific research.

Time-Restricted Eating

A mouse allowed to eat 24 hours a day (left) had much higher levels of liver fat (white) than one that consumed the same high-fat diet within an 8-hour daily feeding window (right).
Photo Credit: Courtesy of the Salk Institute for Biological Studies

Mice that eat only during certain hours avoid obesity and related health problems—even on a high-fat diet.

Transcript

Eating against the clock. I’m Bob Hirshon and this is Science Update.

Putting more hours between dinner and breakfast may stave off obesity, at least in mice. This according to biologist Satchin Panda, and his colleagues at the Salk Institute in La Jolla, California. His team studied mice on high-fat diets. One group was allowed to eat freely at all times, and became obese and unhealthy. But another group, which ate the same number of daily calories as the first group—but within a strict eight-hour window—fared better.

Panda:Surprisingly, the mice that were eating the same number of calories, but ate only for eight hours, did not become as obese as the first group. Less than 12 percent of their body weight was fat, as opposed to 40 percent.

That group’s cholesterol, blood sugar, and liver function were all nearly normal as well. Panda says that modern society’s late-night dining and snacking may be contributing to the obesity epidemic. I’m Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

The possible causes of today's obesity epidemic are many and complex. They include lifestyle factors, like decreases in exercise and sleep. Labor-saving technologies and the Internet have reduced our need to even stand up or walk around. We're also consuming more and more processed, high-calorie foods and beverages in increasingly large portions.

When it comes to the effect of an individual's diet, there are different schools of thought. Some experts maintain that it comes down to calories in, calories out. In other words, you'll gain weight in proportion to how much you overeat, plain and simple. However, many others believe that there are other factors in play, including what types of foods the calories come from, and when and how we eat them.

This study addresses one possibility: that eating around the clock has made us fatter. Now, clearly, very few people eat at a constant rate 24/7; sleep and daily activities still shift our eating to certain times of day. However, the authors of this study suggest that compared to our ancient ancestors, and even to people in relatively recent times, we're eating across more hours of the day than ever before. Could adding mid-morning muffins, afternoon treats, late-night dinners, and midnight snacks to a traditional three-meal schedule be adding to our weight—even if we manage not to take in more calories?

Studying this pattern in people would be challenging, because it's hard to precisely control people's eating habits over a long period of time. However, captive mice are good models for humans. The researchers actually divided the mice into four groups, or to be precise, two groups each divided in two. Half the mice were fed a diet with 60 percent of its calories from fat, while the other half, a control group, ate normal, healthy food. Within each of those groups, half the mice ate on an unrestricted schedule, while the other half were allowed to eat only within an eight-hour window. (According to the researchers, those mice quickly learned that food was available only at certain times, and consumed all the food they were given within that time period.)

After 100 days, the differences in mice on the high-fat diet were striking. Those that ate whenever they wanted suffered predictable effects. They gained weight and developed high cholesterol, high blood glucose, liver damage, and impaired motor control. However, the mice that ate the same high-fat diet, with the same daily calories, but during a short feeding time, were much better off. They weighed 28 percent less than the free-feeding mice (in a human, for example, that's the difference between 200 pounds and 144 pounds). They also showed virtually none of the ill health effects that went along with the weight gain. On an exercise test, they outperformed not only the overweight mice but also those that had been fed a normal diet. (Within the normal-diet group, the effect of feeding schedule was much weaker.)

Why would this be? According to the researchers, constant eating may deprive our bodies of an important chance to maintain itself. While we eat, the body stores fat, which adds weight and puts stress on the liver, and produces glucose, which elevates blood sugar levels—a sign of diabetes. In contrast, evidence suggests that when we stop eating for several hours, the liver stops releasing glucose into the blood, and instead uses it to repair cellular damage. It also releases enzymes that break down cholesterol into acids, which in turn help break down brown fat—a “good” fat that converts calories into heat.

Without adequate time away from eating, however, this process may be cut short or suppressed entirely. It's not yet clear whether there's a minimum fasting time for the metabolic benefits to kick in at all, or whether they simply work better the longer the fasting time. The researchers also caution that the study shouldn't motivate anyone to adjust their eating schedule and then completely ignore the fat content of their diet. Even if the findings apply to humans, there are many other important reasons to eat a nutritious diet besides maintaining a healthy weight.

Now try and answer these questions:

Describe the differences between the two eating schedules in the experiment.

Why is it important to compare mice that ate the same number of daily calories?

What are some of the possible benefits of a longer overnight fast?

If you wanted to test this in people, how would you design the study? What challenges would you face?